How are the ages of the Earth and universe calculated?
Many independent measurements have established that the Earth and the universe are billions of years old.
Astronomers and geologists have determined that the universe and Earth are billions of years old. This conclusion is not based on just one measurement or one calculation, but on many types of evidence. Here we will describe just two types of evidence for an old Earth and two types of evidence for an old universe; more types can be found under further reading. These methods are largely independent of each other, based on separate observations and arguments, yet all point to a history much longer than 10,000 years. As Christians, we believe that God created the world and that the world declares his glory, so we can’t ignore what nature is telling us about its history.
Age of the Earth from seasonal rings and layers
If you’ve ever seen a horizontal slice of a tree trunk, you’ve seen how a tree forms a new growth ring each year. In years of drought, the tree grows less quickly so the ring is narrower; in good growing seasons the ring is thicker. A tree’s age can be found by simply counting its rings. By comparing the pattern of thick and thin rings to weather records, scientists can verify that the method is accurate. This method can even be used on dead trees that fell in a forest long ago. For example, the last 200 rings in the dead tree might match up with 200 rings early in the life of the living tree, so the two trees together can count back many years.
In this way, multiple trees can be used to build a master chronology for a forested region. European oak trees have been used to build a 12,000-year chronology.1 The annual ice layers in glaciers provide a similar method that goes back much further in history. Each year, snowfall varies throughout the seasons and an annual layer is formed. By carefully extracting ice “cores” (like coring an apple) of glaciers, scientists see that more than 50,000 years of past history are plainly visible. This method can be verified and extended by comparison to historical records for weather, as well as to records of volcanic eruptions around the globe that left thin dust layers on the glaciers. Below the visible layers of snowfall, by comparing chemical isotopes with other studies, scientists have drilled ice cores deep into glaciers and found ice that is 123,000 years old in Greenland2 and 740,000 years old in Antarctica.3 These annual layers go back farther than the 10,000 years advocated by Young Earth Creationists, and the isotope studies reveal ice that is much much older. The Earth is at least 740,000 years old.
Age of the Earth and solar system from radiometric dating
In your high school science classroom, you may have seen a large poster of the periodic table hanging on the wall. The periodic table shows the types of atoms that make up the world around us. An element in the periodic table can come in different flavors called isotopes. Some isotopes are unstable, and over time these isotopes “decay” into isotopes of other elements. For example, Potassium-40 is unstable and decays into Argon-40. As time passes, a rock will have more and more Argon-40 and less and less Potassium-40. Radiometric dating is possible because this decay occurs at a known rate, called the “half-life” of the radioactive element. The half-life is the time that it takes for half the radioactive sample to change from one element into the other.
Some isotopes have short half-lives of minutes or years, but Potassium-40 has a half-life of 1.3 billion years. Radiometric dating requires that one understand the initial ratio of the two elements in a given sample by some means. In this case, Argon-40 is a gas that easily bubbles out and escapes when it is produced in molten rock. Once the rock hardens, however, all the Argon-40 is trapped in the sample, giving us an accurate record of how much Potassium-40 has decayed since that time. So, if we find a rock with equal parts Potassium-40 and Argon-40, we know that half the Potassium-40 has decayed into Argon-40, and that the rock hardened 1.3 billion years ago.4
It’s hard to find rocks on the surface of the Earth that have not been altered over time. Most old rocks have been eroded by wind and water or submerged by continental plates. The oldest reliably dated rock formation is in Greenland, where several different isotopes were used to find an age of 3.6 billion years.5 Scientists also recently dated zircon grains (which resist erosion) in Western Australia to 4.4 billion years old.6 To find older rocks that haven’t been eroded, we need to look beyond Earth. Meteorites are rocks from the solar system that have fallen to Earth recently and haven’t suffered much erosion. Their pristine interiors give an age that dates back to their formation at the beginning of the solar system. Nearly all meteorites have the same radiometric age, 4.5 billion years old.7 Thus, the solar system, including the Earth, is about 4,500,000,000 years old.
Age of galaxies from the travel time of light
What about the ages of stars and galaxies, and the age of the whole universe? One way to measure these ages is with the travel time of light. Light travels incredibly fast – 300,000 kilometers per second, or 186,000 miles per second. On Earth, the delay due to light travel time is a tiny fraction of a second. But in space, the distances are so vast that the light takes a substantial amount of time to travel to us: 8.3 minutes from the Sun, 4.3 years from the nearest star, and about 8500 years from the center of the Milky Way galaxy. That delay means that we don’t see these objects as they are right now, but as they were when the light left. The universe actually works as a sort of “time machine,” in which we can see into the past simply by looking far away.
The calculation of the light travel time is simple once you know the speed of light and have a measurement of the distance. The speed of light is well known from experiments on Earth, and various astronomical observations confirm that the speed of light has not changed over the history of the universe. But measuring distances in astronomy is not trivial – you can’t just string a measuring tape from here to the center of the galaxy! Instead, astronomers use several interlocking methods to determine the distances, such as geometric calculations and brightness measurements. For example, some galaxies look much smaller and fainter than other galaxies of the same kind, showing they are much further away.8
The Andromeda galaxy, a near neighbor to our own Milky Way galaxy, is 2.3 million light years away. That is, we are seeing it as it was 2.3 million years ago. But that is just our local neighborhood. In recent decades, astronomers have detected galaxies located several billion light years away. If the light has been traveling billions of years to reach us, then the universe must be at least that old. This is completely independent of radiometric dating of the solar system, but both methods point to an age of billions of years, not thousands.
Age of the universe from expansion
Not only can astronomers measure the distance of galaxies, they can measure how galaxies are moving. Galaxies are not holding still in space, nor are they moving randomly. Some galaxies are moving towards their neighbors, attracted by their mutual gravity. But the biggest pattern we see is that galaxies are moving apart from one another. This motion apart is not all at the same speed; instead it follows a pattern where galaxies that are further apart are moving more quickly.
This particular pattern indicates the whole universe is expanding. To see why, consider a loaf of raisin bread. The raisins are like galaxies and the dough is like the fabric of space in the universe. As the dough rises, it carries the raisins along, pulling them apart from each other. Raisins that started out on opposite sides of the loaf will be a few inches farther apart after the dough rises, while raisins that started out near each other may only move half an inch. So, the speed of their motion is proportional to the separation between them. In the same way, the space of the universe pulls galaxies further apart as the universe expands.
Astronomers detect a galaxy’s motion by looking at its light spectrum. When a galaxy is carried away by the expansion of space, its light waves are stretched out, making it appear redder. The change in the galaxy’s color is called the red shift, and can be used to calculate its velocity. From the measurements of many galaxies, astronomers can accurately measure the expansion rate of the universe as a whole.
The age of universe can be determined by imaging what the universe looked like in the past, “rewinding” the expansion. In the past the galaxies must have been closer together, and in the distant past they would have been packed together in a tiny point. If we assume that the expansion rate is constant over time, the age for the universe as a whole is about 10 billion years. However, astronomers have been working over the last 20 years to determine how the expansion rate changes with time. We now know that early in the universe the expansion was slowing down, but now it is speeding up. Using careful measurements of this change in expansion rate, the age of the universe is now known quite precisely to be 13.79±0.02 billion years.9
Many different and complementary scientific measurements have established with near certainty that the universe and the Earth are billions of years old. Layers in glaciers show a history much longer than 10,000 years, and radiometric dating places the formation of the Earth at 4.5 billion years. Light from galaxies is reaching us billions of years after it left, and the expansion rate of the universe dates its age to 13.8 billion years. These are just a sampling of the types of evidence for the great age of the Earth and the universe; see the resources below for more.
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